1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to arc fault circuit interrupters. The invention also relates to methods of detecting and interrupting a resistive series arc of a power circuit.
2. Background Information
Conventional overcurrent protection devices for alternating current (AC) electrical systems, such as circuit breakers and overload relays, typically do not respond to arc faults. Such faults often have a high resistance, in order that the fault current is below the instantaneous trip current of such protection devices. Arc faults in AC systems also tend to be intermittent, such that they do not generate the time integrated values of current needed to activate a delayed trip feature in a typical circuit protection device.
A common approach to arc fault detection recognizes that the arc current is considerably distorted from a pure sine wave. For instance, it contains considerable high frequency noise. In addition, there can be intervals of no current and/or periods of short circuit current. Devices which combine various of these characteristics of arc current have been proposed for arc detectors. Some of them employ microcomputers to apply the various criteria to the current waveform.
Another approach to arc fault detection relies on the fact that an arc between spaced conductors or a gap in a conductor can only be struck when the voltage rises to the break down voltage across the space or gap. Thus, these detectors respond to the step increase in current produced by arc initiation. Such a detector is described in U.S. Pat. No. 5,224,006. In order to avoid false trips produced by the inrush currents at turn on of some typical loads, another detector of this type, as described in U.S. Pat. No. 5,940,256, looks for a plurality of step increases in current within a certain time interval characteristic of an arc fault, which repetitively strikes and is interrupted. A further improvement on this type of detector, which generates a time attenuated integration of pulses representative of step increases in current, such as those generated by arc faults, is described in U.S. Pat. No. 5,691,869.
Arc fault circuit interrupters (AFCIs) are devices intended to mitigate the effects of arc faults by functioning to deenergize an electrical circuit when an arc fault is detected. Non-limiting examples of AFCIs include: (1) arc fault circuit breakers; (2) branch/feeder arc fault circuit interrupters, which are intended to be installed at the origin of a branch circuit or feeder, such as a panelboard, and which may provide protection from ground faults (e.g., greater than 40 mA) and line-to-neutral faults (e.g., greater than 75 A); (3) outlet circuit arc fault circuit interrupters, which are intended to be installed at a branch circuit outlet, such as an outlet box, in order to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing, and which may provide protection from line-to-ground faults (e.g., greater than 75 A) and line-to-neutral faults (e.g., 5 to 30 A, and greater than 75 A); (4) cord arc fault circuit interrupters, which are intended to be connected to a receptacle outlet, in order to provide protection to an integral or separate power supply cord; (5) combination arc fault circuit interrupters, which function as either a branch/feeder or an outlet circuit AFCI; and (6) portable arc fault circuit interrupters, which are intended to be connected to a receptacle outlet and provided with one or more outlets.
FIG. 1A shows the characteristic current 2, which results from a circuit (not shown) in which, for example, a 60 Hz, 120 VRMS source (not shown) is connected in series with a resistive load (not shown), for example, 5 ARMS, and an electrical arc is introduced in series with this circuit by a carbon electrode (not shown) and a copper electrode (not shown) separated by a relatively small gap.
FIG. 1B shows the line-to-neutral source voltage 4 and the arc voltage 6 during the series arc of FIG. 1A. During a short period of time, which coincides with the zero crossings of the sinusoidal voltage source, the voltage in the circuit is too low to initiate and sustain an electrical arc across the gap between the carbon and copper electrodes (not shown). During this time, the arc voltage 6 across the gap between these electrodes is equivalent to the source voltage 4 and no current flows in the circuit. This is shown by the “flat spots” 8 of FIG. 1A.
However, when the source voltage becomes sufficiently great enough, an electrical arc is sustained between the electrodes and current flows in the circuit as shown by characteristic current 2 of FIG. 1A, which occurs between the flat spots 8, such as at 10 or 12. As a result, a characteristic “current shape” is associated with electrical arcs in series with a resistive load. This current shape can be seen to be nearly sinusoidal, but having the flat spots 8 or periods of zero current conduction, which coincide with the zero crossings of the source voltage 4.
There is room for improvement in arc fault circuit interrupters.
There is also room for improvement in methods of detecting and interrupting a resistive series arc of a power circuit.